DESCRIPTION: The investigator has isolated and cloned a novel HIV-1 strain (89.6) that is SI and infects macrophages, lymphocytes and CD4+ T cell lines. The virus uses both M-tropic and T-tropic entry cofactors, CCR5 and CXCR4. The investigator has evidence that determinants, in addition to those needed for CCR5 usage, are required for macrophage infection. In addition, while macrophages are resistant to prototype T-tropic strains which use CXCR4, the investigator found that macrophages express CXCR4 and that it can mediate entry by certain dual-tropic isolates. Thus, HIV-1 strains differ in their use of chemokine receptors in different cell types. The investigator hypothesizes that dual-tropic strains play an important role in pathogenesis and that dual-tropism may result from an isolate that uses both CCR5 and CXCR4 or one that can use CXCR4 on macrophages in addition to cell lines. The investigator will examine the molecular basis for dual-tropism and proposes four specific aims. Identify the determinants of 89.6 dual CCR5 and CXCR4-mediated fusion and tropism. Define the structural basis for utilization of CXCR4 on macrophages. Identify CCR5 and CXCR4 domains important for interaction with specific envelope regions. Determine whether envelope-cofactor-mediated signaling is associated with strain-specific macrophage entry and tropism.